1. Field of the Invention
The present invention relates to a method for preparing a circuit board having 3-dimensional features by electrodeposition. More specifically, the invention is directed to the application of a uniform coating of a photoresist to a metallized substrate having 3-dimensional features.
2. Description of the Prior Art
Injection-molded circuit boards having 3-dimensional features are providing new ways of connecting and mechanically retaining electrical components. Injection molding has proven to be a very cost effective production technique for large volumes of identical precision parts. Thermoplastic resins are easily processed on conventional injection molding equipment, thus taking advantage of the economics of this process.
Three-dimensional circuit boards are not a direct substitution for a conventional planar circuit board. Instead, they are an integration of circuitry, wiring, hardware and physically structure into a one-piece consolidation.
Applications of injection-molded 3-dimensional circuit boards are only limited by the imagination of the electronics industry. Typically, applications include telecommunications, electronic toys and games, automotive, personal computers, to name a few.
Unlike the conventional flat board, the molded circuit board can take on almost any desired shape, and the circuit can be placed almost anywhere desired. Molding these circuit boards out of high temperature thermoplastic substrates such as polyetherimide (PEI) and polyethersulfone (PES) (which are generally filled with glass fibers and or mica) allows soldering without significantly affecting the dimensional stability of the substrate.
One approach for manufacturing 3-D boards involves two steps. In step one, a catalyzed resin containing palladium is injection molded. In a second step, a non-catalyzed resin is selectively injected over the molded form of step one to define the circuitry. While this approach does away with the use of conventional masks to define the circuitry, the process is very expensive and inflexible in that circuitry lines cannot be modified or changed except by creating entirely new molds.
Another approach to preparing 3-dimensional circuit boards involves injection molding followed by metallization, typically with copper. Following metallization a circuit image is formed by coating the copper layer with an organic coating material. The organic coating material may be either a screen resist or a photoresist. Using a screen resist, the image is formed during the coating process such as by silk screening. Using the photoresist the coating is applied as a single unitary coating by spraying, dipping, spin coating, etc. The photoresist film is then exposed to activating radiation in a desired circuit image and the exposed coating is developed with a liquid developer capable of differentiating light exposed areas from areas that have not been light exposed and dissolving one or the other dependent upon whether the photoresist is a positive or a negative photoresist. Examples of suitable photoresist materials are disclosed in U.S. Pat. Nos. 4,093,461; 4,148,654,; and 4,339,516 incorporated herein by reference.
Development of a photoresist bares the underlying metal layer on the surface of the substrate. The metal layer may then be reinforced by electroplating a metal such as copper thereon, which copper comprises the circuit pattern of the finished board. The photoresist used in the process must be able to withstand attack by the electroplating bath. The remaining copper must then be removed to form the circuit. This may be accomplished by electroplating a dissimilar metal over the copper such as solder, immersion tin, gold or a tin nickel alloy. The organic coating in an image pattern permits deposition of the etch resistant metal. The organic coating is then removed with a solvent to bare the remainder of the copper layer. Such solvents are known in the art and may include aqueous formulations dependent upon the photoresist used to define the image pattern. The copper layer is dissolved by contact with an etchant which dissolves exposed copper but does not aggressively attack the etch resistant metal and therefore does not attack the copper protected by the etch metal. Consequently, the etching step permanently alters the surface of the substrate i.e., by removing exposed copper by etching.
A final step in the fabrication of a board may involve stripping (dissolving) the etch resistant metal from the board leaving the desired circuit pattern. A processing sequence such as that discussed above is set forth in Coombs, Printed Circuit Handbood, McGraw Book Company, New York, 1979, chapters 6 and 7, incorporated herein by reference.
Liquid-type photoresists typically contain a combination of a film forming resin or polymer and a photosensitive compound or a photoinitiator dissolved or suspended in a solvent such as an organic liquid.
Liquid-type photoresists can be negative-acting or positive-acting systems. In the case of a negative-acting photoresist or negative resist, after the film is deposited on a surface and the solvent is removed as by heating, the film is selectively exposed, typically through a photomask, to a source of energy, such as ultraviolet light. The photomask has areas that are opaque and other areas that are transparent to the exposing radiation. The pattern on the photomask formed by the opaque and transparent areas defines the desired image, such as, a circuit, which is to be transferred to the substrate surface. The exposed portions of a negative resist film become less soluble in a developing solution, as the result of a photochemical reaction between the photoinitiator and the polymer or resin upon exposure, than the unexposed portions. This difference in solubility allows for the selective removal of the unexposed film and the transfer of the image to the surface. In a positive resist, the exposed portions of the film become more soluble in the developer than the unexposed portions, as a result of the photochemical reaction, allowing for the selective removal of the exposed areas. After either type of resist film is developed, the portions of the surface that are not protected by the resist may be etched, such as by the action of an oxidizing solution typically containing an inorganic acid. The remaining resist film may then be stripped from the surface leaving only the desired etched image on the substrate. Alternatively, the substrate surface containing the imaged resist can be plated with a metal or combination of metals, such as, tin or tin-lead alloy. The resist may then be selectively stripped and the exposed metal on the substrate may be etched to form the desired pattern or circuit on the substrate surface. The historical background, types and operation of conventional photoresists is described in Photoresist Material and Processes, W. D. DeForest, McGraw-Hill, 1975.
Although liquid-type resists have been used for many years in lithographic and electronic applications, and despite numerous improvements to the resist systems and processing steps involved with their use, these conventional liquid resists still suffer from one or more disadvantages. The most widely recognized drawback of liquid resists, however, has been difficulty to deposit films of uniform and adequate thicknesses on surfaces without the formation of voids or pinholes. For 3-dimensional boards, this is also the case, particularly at holes and corners, which leads to different exposure and developing times and often results in defective products.
Another type of photoresist is referred to as dry film. While dry film resists have been found used in the preparation of 2-dimensional boards, they do not lend themselves to application in 3-dimensional systems because dry film is generally applied using heat and pressure with pinch rollers which do not readily conform to a grossly 3-dimensional features.